Managing Bidirectional Communication in Constrained Environments

ABSTRACT

Bidirectionally transmitting data between a first transceiver and a second transceiver over a transmission line includes communicating between the first transceiver and the second transceiver during a bidirectional communication phase within time slots. The time slots include: a plurality of upstream time slots in which the first transceiver transmits data to the second transceiver, a plurality of downstream time slots in which second transceiver transmits data to the first transceiver, wherein the downstream time slots are shorter than the upstream time slots, and a plurality of idle time slots separating adjacent upstream and downstream time slots.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to and the benefit of U.S. ProvisionalApplication Patent Ser. No. 62/816,767, filed on Mar. 11, 2019, theentire disclosure of which is hereby incorporated by reference.

TECHNICAL FIELD

This disclosure relates to managing bidirectional communication inconstrained environments.

BACKGROUND

Communications over wired media can be defined based on standardizedprotocols. One family of such protocols is Ethernet, as defined by theIEEE 802.3 working group. These standards cover aspects of the Physical(PHY) layer (also known as “layer 1”) associated with the physicalcommunication medium, and medium access control (MAC) layer, which is asublayer of the data link layer (also known as “layer 2”) below thelogical link control (LLC) sublayer. Various versions of the Ethernetstandard are designed to operate at different speeds, and to usedifferent types of physical communication media. For example, someversions of Ethernet that operate at 100 MBits per second (called“100BASE-TX”) are designed for use with a cable that includes separatetwisted pair transmission lines for each direction (upstream anddownstream). Some versions Ethernet that operate at 1 Gbit per second(called “1000BASE-T1”) are designed for use with a single twisted pairtransmission line used for both upstream and downstream directions,using echo cancellation to mitigate the impact of reflections on thetransmission line for optimization of the available bandwidth, which isshared equally for upstream and downstream directions.

The description above is presented as a general background relevant tothis technical field and should not be construed as an admission thatany of the information it contains constitutes prior art against thepresent patent application.

SUMMARY

In one aspect, in general, a networking system for bidirectionallytransmitting data comprises: a communication medium comprising atransmission line having a first end and a second end; a first nodecomprising a first transceiver coupled to the transmission line inproximity to the first end through a first bidirectional interface thatis configured to transmit electromagnetic signals to the transmissionline and receive electromagnetic signals from the transmission lineduring a bidirectional communication phase; and a second node comprisinga second transceiver coupled to the transmission line in proximity tothe second end through a second bidirectional interface that isconfigured to transmit electromagnetic signals to the transmission lineand receive electromagnetic signals from the transmission line duringthe bidirectional communication phase. The first node and the secondnode are configured to communicate during the bidirectionalcommunication phase within time slots that include: a plurality ofupstream time slots in which the first transceiver transmits data to thesecond transceiver, a plurality of downstream time slots in which secondtransceiver transmits data to the first transceiver, wherein thedownstream time slots are shorter than the upstream time slots, and aplurality of idle time slots separating adjacent upstream and downstreamtime slots.

Aspects can include one or more of the following features.

The first node further comprises: a port configured to provide collecteddata from a sensor, and a first medium access controller configured tocontrol timing of access by the first transceiver to the transmissionline during the bidirectional communication phase; and the second nodefurther comprises a second medium access controller configured tocontrol timing of access by the second transceiver to the transmissionline during the bidirectional communication phase.

The networking system further comprises the sensor, in communicationwith the port of the first node, and configured to collect at least aportion of the collected data during the bidirectional communicationphase.

During a plurality of the upstream time slots, the first transceivertransmits data that includes at least some of the collected data to thesecond transceiver.

The first and second medium access controllers are configured to providea minimum duration of the idle time slots is based at least in part on atermination characteristic of the transmission line.

The first end is terminated with an impedance configured to reducereflections from the first end by at least 80%, and the second end isterminated with an impedance configured to reduce reflections from thesecond end by at least 80%.

The first and second medium access controllers are configured to providethe minimum duration to be long enough for a reflection from the firstend or second end to propagate over a round trip between the first endand second end at least twice.

The first medium access controller and the second medium accesscontroller are configured to exchange information by the firsttransceiver and the second transceiver, respectively, during a setupphase, and the setup phase includes transmitting between the second nodeand the first node a value indicating at least one of: a selected timeduration of the upstream time slots or the downstream time slots, or aselected time slot ratio between the upstream time slots and thedownstream time slots.

The first node includes a first timing module configured to provide afirst timing signal for determining time slots in which to transmit andreceive using the first transceiver, and the second node includes asecond timing module configured to provide a second timing signal fordetermining time slots in which to transmit and receive using the secondtransceiver.

The first node and the second node are configured to synchronize thefirst timing module to the second timing module during the setup phase.

The second node further comprises a communication port that is incommunication with a computing device configured to process at least aportion of the collected data.

The second node comprises a networking switch configured to route atleast a portion of the collected data to the computing device.

The first medium access controller and the second medium accesscontroller are configured to provide a communication session in whichall downstream time slots have the same duration, and all the upstreamtime slots have the same duration.

A vehicle includes the networking system configured to transmit databetween devices disposed in different portions of the vehicle.

In another aspect, in general, a method is used for bidirectionallytransmitting data between a first transceiver and a second transceiverover a transmission line through respective bidirectional interfacesthat transmit electromagnetic signals to the transmission line andreceive electromagnetic signals from the transmission line. The methodcomprises: communicating between the first transceiver and the secondtransceiver during a setup phase in which information is exchangedbetween the first transceiver and the second transceiver, theinformation including a value indicating at least one of: a selectedtime duration of upstream time slots or downstream time slots, or aselected time slot ratio between upstream time slots and downstream timeslots; and communicating between the first transceiver and the secondtransceiver during a bidirectional communication phase within time slotsdetermined according to the value. The time slots include: a pluralityof upstream time slots in which the first transceiver transmits data tothe second transceiver, a plurality of downstream time slots in whichsecond transceiver transmits data to the first transceiver, wherein thedownstream time slots are shorter than the upstream time slots, and aplurality of idle time slots separating adjacent upstream and downstreamtime slots.

Aspects can include one or more of the following features.

The method further comprises receiving collected data through a port ofa first node that includes the first transceiver.

The method further comprises, during a plurality of the upstream timeslots, transmitting data that includes at least some of the collecteddata from the first transceiver to the second transceiver.

Communicating between the first transceiver and the second transceiverduring the bidirectional communication phase includes waiting during theidle time slots for a minimum duration of the idle time slots that islong enough for a reflection from a first end or second end of thetransmission line to propagate over a round trip between the first endand the second end at least twice.

The method further comprises, during the setup phase, synchronizing afirst timing signal for determining time slots in which to transmit andreceive using the first transceiver, and a second timing signal fordetermining time slots in which to transmit and receive using the secondtransceiver.

The synchronizing comprises locking the second timing signal to thefirst timing signal.

Aspects can have one or more of the following advantages.

Ethernet is an example of a form of communication that can operate inconstrained environments, such as an automotive environment, in whichcertain operating requirements (e.g., electromagnetic noiserequirements, and/or temperature requirements) must be met, and certainphysical constraints (e.g., cable length) can be assumed. However, someprevious Ethernet protocols have had characteristics that presentchallenges to achieving greater optimization in some constrainedenvironments. Some implementations of various techniques describedherein are able to mitigate some or all of these potential challenges,examples of which are described below.

Some Ethernet protocols are configured to operate using separatecommunication media (e.g., separate transmission lines) for upstream anddownstream communication in a point-to-point link. However, in anenvironment such as an automobile, the extra weight that would resultfrom doubling the number of transmission lines for every link could bedetrimental to satisfying weight constraints. For example, the combinedweight of cables used for hundreds of nodes within an automobile may beon the order of a hundred kilograms or more. Some implementationsdescribed herein use a shared communication medium between nodes oneither end of a link.

Some Ethernet protocols are configured to operate using a symmetricallink that divides the available bandwidth on a shared communicationmedium (e.g., a point-to-point transmission line) evenly. In anenvironment such as an automobile, there may be some nodes (e.g., sensornodes, or nodes sending sensor data upstream from sensor nodes) thathave a much larger volume of data to transmit than other nodes (e.g.,control nodes, or nodes sending control information downstream fromcontrol nodes). In conventional Ethernet systems, components areconfigured to offer balanced bandwidth capacity in both upstream anddownstream directions. Because networking systems are designed toaccommodate the requirements of the direction in which greater capacityis needed, there typically is unused bandwidth capacity in the otherdirection.

In the current disclosure a system is described which departs fromtypical balanced bandwidth capacity designs and instead provides theoption of establishing an uneven (or “asymmetrical”) division of thetotal available bandwidth. In some implementations, this uneven divisioncan be facilitated by communication during a setup phase that allows oneof the nodes to send the other node information indicating how thebandwidth will be divided, as described in more detail below.

Some Ethernet protocols are configured to divide bandwidth over a sharedcommunication medium between nodes on either end of a point-to-pointlink using time division multiplexing. In such protocols, echocancellation typically is used to allow the alternating time slots forupstream and downstream communication to be close together without beingdistorted by echoes from reflections between the ends of a transmissionline that are not terminated by a perfectly impedance matched load.However, in an environment such as an automobile, the extra weight,cost, power consumption, and/or complexity that would result from usingsuch echo cancelation circuitry may be detrimental. Instead of echocancellation, some implementations incorporate idle time slots betweenactive time slots to allow reflections to propagate between the endsenough times to fall to low enough amplitudes to avoid significantdistortion. The constrained size of an automotive environment ensuresthat the time needed for the reflections to travel the necessarymultiples of the round trip distance, and the resulting idle time slots,are small enough to consume a relatively small amount of the availablebandwidth.

In some cases, echo cancellation calls for allowing enough margin incommunication circuitry for signal strengths to double (e.g., a 1-Vsignal and its reflection adding to 2 V on the transmission line). Byavoiding the need for echo cancellation, another advantage may be theability to use the extra signal strength margin for increasing thestandard signal amplitude (e.g., 1.5 V instead of 1.0 V). Thisadditional signal strength can be a significant improvement in anenvironment such as an automobile that may have a significant level ofelectromagnetic noise.

Other features and advantages will become apparent from the followingdescription, and from the figures and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure is best understood from the following detaileddescription when read in conjunction with the accompanying drawings. Itis noted that, according to common practice, the various features of thedrawings are not to-scale. On the contrary, the dimensions of thevarious features are arbitrarily expanded or reduced for clarity.

FIG. 1 is a schematic diagram of an example vehicle.

FIG. 2 is a schematic diagram of an example communication link in acommunications network in the vehicle.

FIG. 3 is a plot of an example communication pattern between an upstreamand downstream node in the communications network.

FIG. 4 is a flowchart of an example procedure for bidirectionalcommunication in the communications network.

DETAILED DESCRIPTION

FIG. 1 is pictorial diagram showing an example of vehicle 100 in whichvarious aspects, features, and elements described herein are implementedin accordance with an embodiment of this disclosure. The vehicle 100includes a communications network (or simply “network”) that enablescommunication among different subsystems in the vehicle 100. The networkmay have any of a variety of network topologies. In someimplementations, some nodes can be connected over a shared communicationmedium, such as a bus, to which different nodes are connected. In someimplementations, certain nodes (e.g., sensor nodes) are connected toother nodes (e.g., a switch node) over a point-to-point link. In theexample of FIG. 1, the network includes multiple network segmentsconfigured to provide for communication and control over multiple zoneswithin the vehicle 100. A front zone network segment 102A includes aswitch 104A that serves as a gateway to the network segment 102A, amiddle zone network segment 102B includes a switch 104B that serves as agateway to the network segment 102B, and a back zone network segment102C includes a switch 104C that serves as a gateway to the networksegment 102C. In this example, the front zone network segment 102A andthe back zone network segment 102C include respective control modules106A and 106B that provide control functionality for various subsystemsin the vehicle 100 over the network. For example, the control module106A is connected to a network controller 108 that couples the controlmodule 106A to the network through the switch 104A. The networkcontroller 108 is configured to communicate using a communicationprotocol (e.g., an Ethernet type protocol), as described in more detailbelow. In this example, the middle zone network segment 102B isconfigured to enable communication with various sensor devices, but doesnot include a dedicated control module.

The control modules can be implemented, for example, using asystem-on-a-chip (SoC) or other electronic circuitry that includes oneor more processor cores. A given processor core can be configured as ageneralized unit such as a central processing unit (CPU), a specialpurpose unit such as a graphics processing unit (GPU), and/or other formof processing circuitry. For example, the control modules can beimplemented using one or more special purpose processors, one or moredigital signal processors, one or more microprocessors, one or morecontrollers, one or more microcontrollers, one or more integratedcircuits, one or more Application Specific Integrated Circuits (ASICs),one or more Field Programmable Gate Arrays, one or more programmablelogic arrays, one or more programmable logic controllers, one or morestate machines, or any combination thereof. The control modules canexecute programs based on stored code, including code stored in storagemodule comprising any tangible non-transitory computer-usable orcomputer-readable medium, capable of, for example, containing, storing,communicating, or transporting machine readable instructions, or anyinformation associated therewith, for use by or in connection with thecontrol module. For example, the storage module can include any form ofvolatile or non-volatile memory including one or more solid statedrives, one or more memory cards, one or more removable media, one ormore read-only memories, one or more random access memories, one or moredisks, including a hard disk, a floppy disk, an optical disk, a magneticor optical card, or any type of non-transitory media suitable forstoring electronic information, or any combination thereof.

Examples of the kind of sensor devices that can be included, in any ofthe zones of the vehicle, include an imaging/navigation sensor 110A(e.g., video camera, radar, LiDAR, rangefinders or other proximitysensors, velocity sensors, accelerometers, infrared-sensing,acoustic-sensing (including ultrasonic sensors), GPS, etc.), and anenvironmental/user-interface sensor 110B (e.g., mass air flow, enginespeed, acceleration, braking, traction, oxygen, fuel temperature,pressure, voltage, steering wheel position, seating position, eyetracking, etc.). The imaging/navigation sensor 110A provides data to thenetwork through a communication bridge 112A, and theenvironmental/user-interface sensor 110B provides data to the networkthrough a communication bridge 112B. These communication bridges can beconfigured according to a communication protocol to facilitatetransmission of potentially large amounts of sensor data to one or bothof the control modules, as described in more detail below. While thisexample shows some sensors connected to one of the control modulesthrough two or more point-to-point links via one or more switches, othersensors can be connected to a control module over a singlepoint-to-point link without going through any intervening switches,which may be used for “edge processing” that avoids moving large amountof data across the vehicle network, potentially reducing networkcongestion.

There are typically a large number of subsystems of the vehicle that canalso be configured to provide data to, and/or be controlled by, one ormore of the control modules. For example, some of these subsystems maybe associated with the vehicle's chassis, wheels, or powertrain (e.g.,including a power source, suspension, drive shaft, axles, and exhaustsystem). The power source, such as an internal combustion engine, anelectric motor, or a combination of an internal combustion engine and anelectric motor, may be operative to provide kinetic energy as a motiveforce to one or more of the wheels. There may also be a large number ofdriver controls (e.g., for power-up/ignition, steering, acceleration,and braking) and displays, or other user interface input and outputelements, that are in communication with the network. Also, alternativetypes of vehicles, other than automobiles, can include other subsystemsincluding subsystems for other types of propulsion, such as a propellersfor aerial vehicles. Any of a variety of modules of these subsystems canbe electronically controllable modules that are controlled, at least inpart, based on signals sent to or from one or more of the controlmodules. Communication nodes associated with these subsystems indifferent network segments communicate with other subsystems, such asthe powertrain, the wheels, or both, for example, to control the vehicle100, such as accelerating, decelerating, steering, or otherwisecontrolling the vehicle 100.

There may be various types of communication media that connect differentnodes of the network. The switches and communication bridges can beinterconnected by cables for transmitting encoded signals (e.g., signalsencoded using amplitude and/or phase of a transmitted wave according toa suitable protocol). In some cases, there may be different types ofcables between different types of nodes, some of which may havedifferent physical characteristics such as length, bandwidth capacity,and/or shielding materials. For example, one type of cable 114 may beused between a sensor bridge and a switch, and another type of cable 116may be used between different switches. The cables can include one ormore communication media such as electrical wiring and/or optical fiber.The switches and communication bridges can be configured to includecircuitry for providing appropriate functionality according toparticular communication protocols in a layered protocol stack. Forexample, a PHY layer protocol can be used by a transceiver that includescircuitry for transmitting signals onto a communication medium andcircuitry for receiving signals from the communication medium. In someimplementations, there is separate transmitter circuitry and receivercircuitry, and circuitry to control whether the transmitter or receiveris actively accessing the communication medium. A MAC layer can be usedby a medium access controller that controls access by the transceiver tothe communication medium. Medium access controllers, and/or othercircuitry controlling timing of the transceivers (e.g., circuitry ineither or both of the MAC layer and/or PHY layer), are able to configurelink 118 between the switch 104B and the communication bridge 112B toprovide an uneven division of the available bandwidth for largequantities of sensor data to efficiently flow upstream. Other types ofcables can also be included in the vehicle 100, such as cables fordelivering electrical power. For example, the control switches, controlmodules, and sensors can be configured to receive power from thepowertrain over a power delivery network.

FIG. 2 is a schematic showing an example of a communication link betweentwo nodes of a network in a constrained environment such as the vehicle100 of FIG. 1. An electrical transmission line 200 (e.g., a twisted pairincluding two conducting wires surrounded by insulating material twistedtogether) provides a shared point-to-point connection between adownstream node 202A and an upstream node 202B. The downstream node 202Aincludes a transceiver 204A coupled to the end of (or in proximity tothe end of) the transmission line 200, and the upstream node 202Bincludes a transceiver 204B coupled to the end of (or in proximity tothe end of) the transmission line 200. Each transceiver includes abidirectional interface that includes transmitter circuitry configuredto transmit electromagnetic signals (e.g., voltage signals) to thetransmission line 200, and receiver circuitry configured to receiveelectromagnetic signals from the transmission line 200. The transceiversand interface circuitry coupling the transceivers to the transmissionline 200, are in some implementations, integrated within a PHY deviceconfigured to conform to a physical layer standard. The downstream node202A includes a medium access controller 206A configured to control thetransceiver 204A, and the upstream node 202B includes a medium accesscontroller 206B configured to control the transceiver 204B. In thisexample, the downstream node 202A is in communication with a sensor 210,either directly over a bridge or indirectly over one or more additionallinks through one or more switches; and the upstream node 202B is incommunication with a processor 212 (such as a processor in a controlmodule), either directly through a network controller or indirectly overone or more additional links through one or more switches.

Due to the quantity of data that is collected by the sensors and thespeed at which that data needs to arrive at an upstream node in thenetwork, such as a control module or processor, it is useful for atleast some of the links in the network to be configured to provide asignificantly larger share of the available bandwidth for the upstreamdirection, and a smaller share in the downstream direction. So, theprotocol used by the nodes 202A and 202B can be designed to provide alarger share of the bandwidth from the downstream node 202A to theupstream node 202B, as shown. FIG. 3 is a ladder diagram showing anexample of a communication pattern 300 between an upstream node 302A anddownstream node 302B. Sensor data 304 is received at the downstreamnode, such as a sensor device, and may be temporarily stored or bufferedin a storage device (e.g., volatile and/or non-volatile memory) as it issent upstream, or streamed directly without requiring storage of anysignificant amount of the sensor data 304. In some cases a sensor deviceis collecting at least a portion of the sensor data 304 duringbidirectional communication in which that sensor data is being sentupstream in an efficient manner (e.g., without wasting bandwidth thatwould otherwise be unused).

After the downstream node 302B receives sensor data 304, there is a linksetup phase, which in this example occurs over an initial time period306. The downstream node 302B initiates (308) the link setup in thisexample, but in other examples the upstream node 302A may initiate thelink setup. In some implementations, the nodes are configured with onenode (e.g., the downstream node) acting as a “master” node and anothernode (the upstream node) acting as a “slave” node, where the master nodeinitiates link setup. The setup phase may proceed according to a“handshake” protocol in which various characteristics of the link areestablished. In this example, those characteristics include thedurations of a set of alternating downstream and upstream time slots.Time slot information 310 includes a value indicating a selected timeduration of the upstream time slots and the downstream time slots.Alternatively, one of the time slot durations (e.g., the longer upstreamtime slot) can be predetermined and the other time slot duration (e.g.,the shorter downstream time slot duration) can be determined by themaster node and sent to the slave node. Alternatively, the master nodecan determine and send a selected time slot ratio between the upstreamtime slots and the downstream time slots. The ratio can be configurableto allow different levels of bandwidth sharing in different cases (e.g.,2:1, 50:1, 100:1, 1000:1, etc.), including an even split of thebandwidth (i.e., a 1:1 time slot ratio) in some cases. The ratio may bedifferent for different links, and/or may change for different sessionsover the same link.

Other link configuration can be performed during the setup phase. Forexample, the master node can send the slave node a signal encoded with atraining sequence. Also, the nodes can perform a synchronizationprocedure to synchronize a timing module at one node to a timing moduleat the other node. For example, the slave node can perform a procedureto lock a timing signal of a slave timing module 214B (e.g., a clockused by the transceiver 204B) to timing signal of a master timing module214A (e.g., a clock used by the transceiver 204A). The locking of aclock can include locking a frequency of the clock. In someimplementations, the setup phase includes minimal activity (e.g.,reducing the number of acknowledgement messages or other unnecessarycommunication) in order to reduce the latency of communication withinthe vehicle enabling quick responses for safety reasons.

In this example, the time over which the link is active is divided intothree kinds of time slots. A downstream time slot 312 has a long enoughduration to provide sufficient bandwidth for command and controlinformation to be sent from a control module. An upstream time slot 316has a long enough duration to reserve a significant share of thebandwidth for sending sensor data from a sensor device (e.g., in somecases more than 90% of the bandwidth, or more than 99% of thebandwidth). In some implementations, the upstream bandwidth is 10 Gbpsand the downstream bandwidth is 100 Mbps. The downstream and upstreamtime slots alternate, with an idle time slot 314 between each of thedifferent types of time slots. The sensor data 304 can be sent from theupstream node 202B to the processor 212, possibly via one or moreadditional links over the network, after each upstream time slot, or canbuffer the sensor data 304 to be sent after the session is over or aftera predetermined number of time slots or predetermined amount of time.

A purpose for the idle time slot 314 is to allow enough time forreflections from imperfectly terminated ends of the transmission line200 to attenuate to low enough amplitudes. In some other systems,reflections are dealt with in other ways, such as by performing echocancellation at the interfaces between the transceivers and thetransmission line. But, the additional complexity, cost, and weight thatwould be called for to implement echo cancellation may not beappropriate in some systems. Allowing enough time for reflections tofade away is not always practical for a communication protocol that isdesigned to be used in a variety of environments. But, if certainconstraints on the environment are assumed in advance, such as a maximumlength requirement on the transmission line (e.g., less than 50 meters,or less than or equal to 15 meters), then a communication protocol canbe optimized for such a size-constrained environment.

Signal distortions can be caused when communicating on a transmissionline due to reflections (also called “echoes”) from imperfectlyterminated ends of the transmission line. For example, a node may havean interface of a transceiver or other physical layer component that isconnected to an end of a transmission line characterized by a certainimpedance. When that impedance is substantially equal to thecharacteristic impedance of the transmission line, then that end of thetransmission line is said to be “impedance matched.” With an impedancematched termination of a transmission line, there is little or no powerreflected from a signal that reaches that end of the transmission line.If one or both ends of the transmission line are not perfectly impedancematched, then there will be a reflection that propagates in the oppositedirection to the other end of the transmission line. Echo cancelers aredesigned to subtract such reflections using additional circuitry at theinterface.

For implementations without an echo canceler (e.g., for reasonsdiscussed above), it is still possible to avoid distortion caused byreflections by enforcing a sufficiently long idle time slot betweenupstream and downstream time slots. For example, if each round trip(i.e., reflection from both ends of the transmission line) reduces thepower in a signal to 10% of its power before that round trip, then 3round trips would yield a power reduction to 0.1%. This level of powerreduction can be achieved in some protocols designed to work in asize-constrained environment that assume a transmission line length of15 m or less with an idle time slot long enough for 3 round trips of 30m. At an assumed transmission line propagation time of about 5 ns/m(slightly larger than the approximately 3.3 ns/m propagation time oflight in a vacuum), the idle time slot should be at least around 450 ns.So, the protocol may allow for the idle time slot 314 to be around0.5-1.0 microseconds.

Referring to FIG. 4, an example of a procedure 400 for bidirectionallytransmitting data between transceivers over a transmission line, asdescribed herein. The procedure 400 includes communicating between afirst transceiver and a second transceiver during a setup phase 402 inwhich information is exchanged between the first transceiver and thesecond transceiver (e.g., according to the MAC layer). The informationexchanged includes a value indicating relative durations of time slots(e.g., at least one of a selected time duration of upstream time slotsor downstream time slots, or a selected time slot ratio between upstreamtime slots and downstream time slots). The procedure 400 includescommunicating between the first transceiver and the second transceiverduring a bidirectional communication phase 404 within time slotsdetermined according to the value. The time slots include upstream timeslots in which the first transceiver transmits (406A) data to the secondtransceiver, and downstream time slots in which second transceivertransmits (406B) data to the first transceiver. In some cases, thedownstream time slots are shorter than the upstream time slots. Thereare also idle time slots separating adjacent upstream and downstreamtime slots during which both transceivers wait (408A) after upstreamtimeslots and wait (408B) after downstream time slots, withouttransmitting over the transmission line. At any point after any numberof rounds of the communication phase 404, such as after a given amountof sensor data has been sent, the MAC layer can terminate thecommunication session between the transceivers.

The techniques described herein can be implemented in a manner that iscompatible with any of a variety of types of protocols, includingprotocol(s) used in a PHY layer, a MAC sublayer, or both, such as anEthernet protocol, or any other protocol that facilitates bidirectionalcommunication over a shared communication medium used for apoint-to-point link. The transmission line can be implemented using anyof a variety of types of cables, which may be selected to have suitablecharacteristics (e.g., shielding, grounding) for a variety ofconstrained environments. For example, a signal can be transmitted as avoltage between a pair of wires in a single-ended configuration in whichone wire is grounded and the other wire carries a voltage with respectto the ground wire to yield a desired voltage signal, or in adifferential configuration in which both wires carry complementaryvoltages whose difference yields a desired voltage signal.

While the disclosure has been described in connection with certainembodiments, it is noted that the disclosure is not to be limited to thedisclosed embodiments but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the scope ofthe appended claims, which scope is to be accorded the broadestinterpretation so as to encompass all such modifications and equivalentstructures as is permitted under the law.

What is claimed is:
 1. A networking system for bidirectionallytransmitting data, comprising: a communication medium comprising atransmission line having a first end and a second end; a first nodecomprising a first transceiver coupled to the transmission line inproximity to the first end through a first bidirectional interface thatis configured to transmit electromagnetic signals to the transmissionline and receive electromagnetic signals from the transmission lineduring a bidirectional communication phase; and a second node comprisinga second transceiver coupled to the transmission line in proximity tothe second end through a second bidirectional interface that isconfigured to transmit electromagnetic signals to the transmission lineand receive electromagnetic signals from the transmission line duringthe bidirectional communication phase; wherein the first node and thesecond node are configured to communicate during the bidirectionalcommunication phase within time slots that include: a plurality ofupstream time slots in which the first transceiver transmits data to thesecond transceiver, a plurality of downstream time slots in which secondtransceiver transmits data to the first transceiver, wherein thedownstream time slots are shorter than the upstream time slots, and aplurality of idle time slots separating adjacent upstream and downstreamtime slots.
 2. The networking system of claim 1, wherein: the first nodefurther comprises: a port configured to provide collected data from asensor, and a first medium access controller configured to controltiming of access by the first transceiver to the transmission lineduring the bidirectional communication phase; and the second nodefurther comprises a second medium access controller configured tocontrol timing of access by the second transceiver to the transmissionline during the bidirectional communication phase.
 3. The networkingsystem of claim 2, further comprising the sensor, in communication withthe port of the first node, and configured to collect at least a portionof the collected data during the bidirectional communication phase. 4.The networking system of claim 2, wherein, during a plurality of theupstream time slots, the first transceiver transmits data that includesat least some of the collected data to the second transceiver.
 5. Thenetworking system of claim 2, wherein the first and second medium accesscontrollers are configured to provide a minimum duration of the idletime slots is based at least in part on a termination characteristic ofthe transmission line.
 6. The networking system of claim 5, wherein thefirst end is terminated with an impedance configured to reducereflections from the first end by at least 80%, and the second end isterminated with an impedance configured to reduce reflections from thesecond end by at least 80%.
 7. The networking system of claim 6, whereinthe first and second medium access controllers are configured to providethe minimum duration to be long enough for a reflection from the firstend or second end to propagate over a round trip between the first endand second end at least twice.
 8. The networking system of claim 2,wherein the first medium access controller and the second medium accesscontroller are configured to exchange information by the firsttransceiver and the second transceiver, respectively, during a setupphase, and the setup phase includes transmitting between the second nodeand the first node a value indicating at least one of: a selected timeduration of the upstream time slots or the downstream time slots, or aselected time slot ratio between the upstream time slots and thedownstream time slots.
 9. The networking system of claim 8, wherein thefirst node includes a first timing module configured to provide a firsttiming signal for determining time slots in which to transmit andreceive using the first transceiver, and the second node includes asecond timing module configured to provide a second timing signal fordetermining time slots in which to transmit and receive using the secondtransceiver.
 10. The networking system of claim 9, wherein the firstnode and the second node are configured to synchronize the first timingmodule to the second timing module during the setup phase.
 11. Thenetworking system of claim 2, wherein the second node further comprisesa communication port that is in communication with a computing deviceconfigured to process at least a portion of the collected data.
 12. Thenetworking system of claim 11, wherein the second node comprises anetworking switch configured to route at least a portion of thecollected data to the computing device.
 13. The networking system ofclaim 2, wherein the first medium access controller and the secondmedium access controller are configured to provide a communicationsession in which all downstream time slots have the same duration, andall the upstream time slots have the same duration.
 14. A vehicle inwhich the networking system of claim 1 is configured to transmit databetween devices disposed in different portions of the vehicle.
 15. Amethod for bidirectionally transmitting data between a first transceiverand a second transceiver over a transmission line through respectivebidirectional interfaces that transmit electromagnetic signals to thetransmission line and receive electromagnetic signals from thetransmission line, the method comprising: communicating between thefirst transceiver and the second transceiver during a setup phase inwhich information is exchanged between the first transceiver and thesecond transceiver, the information including a value indicating atleast one of: a selected time duration of upstream time slots ordownstream time slots, or a selected time slot ratio between upstreamtime slots and downstream time slots; and communicating between thefirst transceiver and the second transceiver during a bidirectionalcommunication phase within time slots determined according to the value,the time slots including: a plurality of upstream time slots in whichthe first transceiver transmits data to the second transceiver, aplurality of downstream time slots in which second transceiver transmitsdata to the first transceiver, wherein the downstream time slots areshorter than the upstream time slots, and a plurality of idle time slotsseparating adjacent upstream and downstream time slots.
 16. The methodof claim 15, further comprising receiving collected data through a portof a first node that includes the first transceiver.
 17. The method ofclaim 16, further comprising, during a plurality of the upstream timeslots, transmitting data that includes at least some of the collecteddata from the first transceiver to the second transceiver.
 18. Themethod of claim 15, wherein communicating between the first transceiverand the second transceiver during the bidirectional communication phaseincludes waiting during the idle time slots for a minimum duration ofthe idle time slots that is long enough for a reflection from a firstend or second end of the transmission line to propagate over a roundtrip between the first end and the second end at least twice.
 19. Themethod of claim 15, further comprising, during the setup phase,synchronizing a first timing signal for determining time slots in whichto transmit and receive using the first transceiver, and a second timingsignal for determining time slots in which to transmit and receive usingthe second transceiver.
 20. The method of claim 19, wherein thesynchronizing comprises locking the second timing signal to the firsttiming signal.